Prevalence of mitochondrial DNA disease in adults

Prevalence of genetic muscle disease in Northern England: in-depth analysis of a muscle clinic population

We have performed a detailed population study of patients with genetic muscle disease in the northern region of England. Our current clinic population comprises over 1100 patients in whom we have molecularly characterized 31 separate muscle disease entities. Diagnostic clarity achieved through careful delineation of clinical features supported by histological, immunological and genetic analysis has allowed us to reach a definitive diagnosis in 75.7% of our patients. We have compared our case profile with that from Walton and Nattrass' seminal study from 1954, also of the northern region, together with data from other more recent studies from around the world. Point prevalence figures for each of the five major disease categories are comparable with those from other recent studies. Myotonic dystrophies are the most common, comprising 28.6% of our clinic population with a point prevalence of 10.6/100,000. Next most frequent are the dystrophinopathies and facioscapulohumeral muscular dystrophy making up 22.9% (8.46/100,000) and 10.7% (3.95/100,000) of the clinic population, respectively. Spinal muscular atrophy patients account for 5.1% or 1.87/100,000 patients. Limb girdle muscular dystrophy, which was described for the first time in the paper by Walton and Nattrass (1954) and comprised 17% of their clinic population, comprises 6.2% of our clinic population at a combined prevalence of 2.27/100,000. The clinic population included patients with 12 other muscle disorders. These disorders ranged from a point prevalence of 0.89/100 000 for the group of congenital muscular dystrophies to conditions with only two affected individuals in a population of three million. For the first time our study provides epidemiological information for X-linked Emery-Dreifuss muscular dystrophy and the collagen VI disorders. Each of the X-linked form of Emery-Dreifuss muscular dystrophy and Ullrich muscular dystrophy has a prevalence of 0.13/100,000, making both very rare. Bethlem myopathy was relatively more common with a prevalence of 0.77/100,000. Overall our study provides comprehensive epidemiological information on individually rare inherited neuromuscular conditions in Northern England. Despite the deliberate exclusion of relatively common groups such as hereditary motor and sensory neuropathy (40/100,000) and mitochondrial disorders (9.2/100,000), the combined prevalence is 37.0/100,000, demonstrating that these disorders, taken as a group, encompass a significant proportion of patients with chronic disease. The study also illustrates the immense diagnostic progress since the first regional survey over 50 years ago by Walton and Nattrass.

Mitochondrial DNA mutations and human disease

Mitochondrial disorders are a group of clinically heterogeneous diseases, commonly defined by a lack of cellular energy due to oxidative phosphorylation (OXPHOS) defects. Since the identification of the first human pathological mitochondrial DNA (mtDNA) mutations in 1988, significant efforts have been spent in cataloguing the vast array of causative genetic defects of these disorders. Currently, more than 250 pathogenic mtDNA mutations have been identified. An ever-increasing number of nuclear DNA mutations are also being reported as the majority of proteins involved in mitochondrial metabolism and maintenance are nuclear-encoded. Understanding the phenotypic diversity and elucidating the molecular mechanisms at the basis of these diseases has however proved challenging. Progress has been hampered by the peculiar features of mitochondrial genetics, an inability to manipulate the mitochondrial genome, and difficulties in obtaining suitable models of disease. In this review, we will first outline the unique features of mitochondrial genetics before detailing the diseases and their genetic causes, focusing specifically on primary mtDNA genetic defects. The functional consequences of mtDNA mutations that have been characterised to date will also be discussed, along with current and potential future diagnostic and therapeutic advances.

Advances in molecular diagnostics for mitochondrial diseases

BACKGROUND

Mitochondrial disorders (MD) are diseases caused by impairment of the mitochondrial respiratory chain. Phenotypes are polymorphous and may range from pure myopathy to multisystemic disorders. The genetic defect can be located on mitochondrial or nuclear DNA. At present, diagnosis of MD requires a complex approach: measurement of serum lactate, electromyography, muscle histology and enzymology, and genetic analysis. Magnetic resonance spectroscopy allows the assessment of tissue metabolic alterations, thus providing useful information for the diagnosis and monitoring of MD. Molecular soluble markers of mitochondrial dysfunction, at rest and during exercise, can identify the impairment of the aerobic system in MD, but a reliable biomarker for the screening or diagnosis of MD is still needed.

OBJECTIVE

Molecular and genetic characterization of MD, together with other experimental approaches, contribute to add new insights to these diseases. Here, the role and advances of diagnostic techniques for MD are reviewed.

CONCLUSION

Possible applications of the results obtained by new molecular investigative approaches could in future guide therapeutic strategies.

Molecular basis of infantile reversible cytochrome c oxidase deficiency myopathy

Childhood-onset mitochondrial encephalomyopathies are usually severe, relentlessly progressive conditions that have a fatal outcome. However, a puzzling infantile disorder, long known as ‘benign cytochrome c oxidase deficiency myopathy’ is an exception because it shows spontaneous recovery if infants survive the first months of life. Current investigations cannot distinguish those with a good prognosis from those with terminal disease, making it very difficult to decide when to continue intensive supportive care. Here we define the principal molecular basis of the disorder by identifying a maternally inherited, homoplasmic m.14674T>C mt-tRNA^Glu mutation in 17 patients from 12 families. Our results provide functional evidence for the pathogenicity of the mutation and show that tissue-specific mechanisms downstream of tRNA^Glu may explain the spontaneous recovery. This study provides the rationale for a simple genetic test to identify infants with mitochondrial myopathy and good prognosis.

Mitochondrial gene replacement in primate offspring and embryonic stem cells

Mitochondria are found in all eukaryotic cells and contain their own genome (mtDNA). Unlike the nuclear genome which is derived from both the egg and sperm at fertilization, the mtDNA in the embryo are derived almost exclusively from the egg, i.e. is of maternal origin. Mutations in mtDNA contribute to a diverse range of still incurable human diseases and disorders. To establish preclinical models for new therapeutic approaches, we demonstrate here that the mitochondrial genome can be efficiently replaced in mature nonhuman primate oocytes by spindle-chromosomal complex transfer from one egg to an enucleated, mitochondrial-replete egg. The reconstructed oocytes with the mitochondrial replacement were capable of supporting normal fertilization, embryo development and produced healthy offspring. Genetic analysis confirmed that nuclear DNA in the three infants born so far originated from the spindle donors while mtDNA came from the cytoplast donors. No contribution of spindle donor mtDNA was detected in offspring. Spindle replacement is shown here as an efficient protocol replacing the full complement of mitochondria in newly generated embryonic stem cell lines. This approach may offer a reproductive option to prevent mtDNA disease transmission in affected families.

Cytochrome c oxidase deficiency: patients and animal models

Cytochrome c oxidase (COX) deficiencies are one of the most common defects of the respiratory chain found in mitochondrial diseases. COX is a multimeric inner mitochondrial membrane enzyme formed by subunits encoded by both the nuclear and the mitochondrial genome. COX biosynthesis requires numerous assembly factors that do not form part of the final complex but participate in prosthetic group synthesis and metal delivery in addition to membrane insertion and maturation of COX subunits. Human diseases associated with COX deficiency including encephalomyopathies, Leigh syndrome, hypertrophic cardiomyopathies, and fatal lactic acidosis are caused by mutations in COX subunits or assembly factors. In the last decade, numerous animal models have been created to understand the pathophysiology of COX deficiencies and the function of assembly factors. These animal models, ranging from invertebrates to mammals, in most cases mimic the pathological features of the human diseases.

The low abundance of clonally expanded mitochondrial DNA point mutations in aged substantia nigra neurons

Clonally expanded mitochondrial DNA (mtDNA) deletions accumulate with age in human substantia nigra (SN) and high levels cause respiratory chain deficiency. In other human tissues, mtDNA point mutations clonally expand with age. Here, the abundance of mtDNA point mutations within single SN neurons from aged controls was investigated. From 31 single cytochrome c oxidase normal SN neurons, only one clonally expanded mtDNA point mutation was identified, suggesting in these neurons mtDNA point mutations occur rarely, whereas mtDNA deletions are frequently observed. This contrasts observations in mitotic tissues and suggests that different forms of mtDNA maintenance may exist in these two cell types.

Phenotypic diversity associated with the mitochondrial m.8313G>A point mutation

We report the clinical, histochemical, and molecular genetic findings in a patient with progressive mitochondrial cytopathy due to the m.8313G>A point mutation in the mitochondrial tRNA(Lys) (MTTK) gene. The clinical features in this case are severe, including short stature, myopathy, peripheral neuropathy, and osteoporosis, while extensive analysis of maternal relatives indicate that the mutation has arisen de novo and was not maternally inherited. This report of a second case, together with single muscle fiber mutation analysis that shows clear segregation of mutation load with cytochrome c oxidase deficiency, confirms that the mutation is pathologic.

mTERF2 regulates oxidative phosphorylation by modulating mtDNA transcription

Regulation of mitochondrial protein expression is crucial for the function of the oxidative phosphorylation (OXPHOS) system. Although the basal machinery for mitochondrial transcription is known, the regulatory mechanisms are not completely understood. Here, we characterized mTERF2, a mitochondria-localized homolog of the mitochondrial transcription termination factor mTERF1. We show that inactivation of mTERF2 in the mouse results in a myopathy and memory deficits associated with decreased levels of mitochondrial transcripts and imbalanced tRNA pool. These aberrations were associated with decreased steady-state levels of OXPHOS proteins causing a decrease in respiratory function. mTERF2 binds to the mtDNA promoter region, suggesting that it affects transcription initiation. In vitro interaction studies suggest that mtDNA mediates interactions between mTERF2 and mTERF3. Our results indicate that mTERF1, mTERF2, and mTERF3 regulate transcription by acting in the same site in the mtDNA promoter region and thereby mediate fine-tuning of mitochondrial transcription and hence OXPHOS function.

Assaying ATP synthesis in cultured cells: a valuable tool for the diagnosis of patients with mitochondrial disorders

Mitochondrial ATP synthase plays a central role in cell function by synthesising most of the ATP in human tissues. In different cells, active regulation of mitochondrial ATP synthase in response to cellular energy demand has been demonstrated, as well as its alteration under several pathological conditions affecting oxidative phosphorylation (OXPHOS). Traditionally, detection of OXPHOS defects is based on the spectrophotometric measurement of respiratory chain complex activities in muscle biopsies. Considering the broad clinical spectrum of mitochondrial disorders, and the difficulty in arriving at a single diagnostic method, in this study we propose measurement of ATP synthesis in mitochondria from skin fibroblasts as an effective screening tool. In the light of our results this assessment emerges as a useful marker of impaired energy production in primary OXPHOS disorders of childhood and as a tool with the potential to drive further molecular genetic studies.

The inheritance of pathogenic mitochondrial DNA mutations

Mitochondrial DNA mutations cause disease in > 1 in 5000 of the population, and ∼ 1 in 200 of the population are asymptomatic carriers of a pathogenic mtDNA mutation. Many patients with these pathogenic mtDNA mutations present with a progressive, disabling neurological syndrome that leads to major disability and premature death. There is currently no effective treatment for mitochondrial disorders, placing great emphasis on preventing the transmission of these diseases. An empiric approach can be used to guide genetic counseling for common mtDNA mutations, but many families transmit rare or unique molecular defects. There is therefore a pressing need to develop techniques to prevent transmission based on a solid understanding of the biological mechanisms. Several recent studies have cast new light on the genetics and cell biology of mtDNA inheritance, but these studies have also raised new controversies. Here we compare and contrast these findings and discuss their relevance for the transmission of human mtDNA diseases.

Endurance exercise is protective for mice with mitochondrial myopathy

Defects in the mitochondrial ATP-generating system are one of the most commonly inherited neurological disorders, but they remain without treatment. We have recently shown that modulation of the peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) level in skeletal muscle of a mitochondrial myopathy mouse model offers a therapeutic approach. Here we analyzed if endurance exercise, which is known to be associated with an increased PGC-1alpha level in muscle, offers the same beneficial effect. We subjected male and female mice that develop a severe mitochondrial myopathy due to a cytochrome-c oxidase deficiency at 3 mo of age to endurance exercise training and monitored phenotypical and metabolic changes. Sedentary myopathy and wild-type mice were used as controls. Exercise increased PGC-1alpha in muscle, resulting in increased mitochondrial biogenesis, and successfully stimulated residual respiratory capacity in muscle tissue. As a consequence, ATP levels were increased in exercised mice compared with sedentary myopathy animals, which resulted in a delayed onset of the myopathy and a prolonged lifespan of the exercised mice. As an added benefit, endurance exercise induced antioxidant enzymes. The overall protective effect of endurance exercise delayed the onset of the mitochondrial myopathy and increased life expectancy in the mouse model. Thus stimulating residual oxidative phosphorylation function in the affected muscle by inducing mitochondrial biogenesis through endurance exercise might offer a valuable therapeutic intervention for mitochondrial myopathy patients.

Inherited mitochondrial optic neuropathies

Leber hereditary optic neuropathy (LHON) and autosomal dominant optic atrophy (DOA) are the two most common inherited optic neuropathies and they result in significant visual morbidity among young adults. Both disorders are the result of mitochondrial dysfunction: LHON from primary mitochondrial DNA (mtDNA) mutations affecting the respiratory chain complexes; and the majority of DOA families have mutations in the OPA1 gene, which codes for an inner mitochondrial membrane protein critical for mtDNA maintenance and oxidative phosphorylation. Additional genetic and environmental factors modulate the penetrance of LHON, and the same is likely to be the case for DOA which has a markedly variable clinical phenotype. The selective vulnerability of retinal ganglion cells (RGCs) is a key pathological feature and understanding the fundamental mechanisms that underlie RGC loss in these disorders is a prerequisite for the development of effective therapeutic strategies which are currently limited.

Batteries not included: diagnosis and management of mitochondrial disease

In 1998, Wallace et al. (Science 1988; 242: 1427-30) published evidence that the mutation m.11778G>A was responsible for causing Leber's hereditary optic neuropathy. This was the first account of a mitochondrial DNA mutation being irrefutably linked with a human disease and was swiftly followed by a report from Holt et al. (Nature 1988; 331: 717-9) identifying deletions in mitochondrial DNA as a cause for myopathy. During the subsequent 20 years there has been an exponential growth in 'mitochondrial medicine', with clinical, biochemical and genetic characterizations of a wide range of mitochondrial diseases and evidence implicating mitochondria in a host of many other clinical conditions including ageing, neurodegenerative illness and cancer. In this review we shall focus on the diagnosis and management of mitochondrial diseases that lead directly or indirectly to disruption of the process of oxidative phosphorylation.

The mitochondrial proteome database: MitoP2

Defining the mitochondrial proteome is a prerequisite for fully understanding the organelles function as well as mechanisms underlying mitochondrial pathology. The core functions of mitochondria include oxidative phosphorylation, amino acid metabolism, fatty acid oxidation, and ion homeostasis. In addition to these well-known functions, many crucial properties in cell signaling, cell differentiation and cell death are only now being elucidated, and with them the proteins involved. With the wealth of information arriving from single protein studies and sophisticated genome-wide approaches, MitoP2 was designed and is maintained to consolidate knowledge on mitochondrial proteins in one comprehensive database, thus making all pertinent data readily accessible (http://www.mitop2.de). Although the identification of the human mitochondrial proteome is ultimately the prime objective, integration of other species includes Saccharomyces cerevisiae, mouse, Arabidopsis thaliana, and Neurospora crassa so orthology between these species can be interrogated. Data from genome-wide studies can be individually retrieved and are also processed by a support vector machine (SVM) to generate a score that indicates the likelihood of a candidate protein having a mitochondrial location. Manually validated proteins constitute the reference set of the database that contains over 590 yeast, 920 human, and 1020 mouse entries, and that is used for benchmarking the SVM score. Multiple search options allow for the interrogation of the reference set, candidates, disease related proteins, chromosome locations as well as availability of mouse models. Taken together, MitoP2 is a valuable tool for basic scientists, geneticists, and clinicians who are investigating mitochondrial physiology and dysfunction.

Mouse models of oxidative phosphorylation dysfunction and disease

Oxidative phosphorylation (OXPHOS) deficiency results in a number of human diseases, affecting at least one in 5000 of the general population. Altering the function of genes by mutations are central to our understanding their function. Prior to the development of gene targeting, this approach was limited to rare spontaneous mutations that resulted in a phenotype. Since its discovery, targeted mutagenesis of the mouse germline has proved to be a powerful approach to understand the in vivo function of genes. Gene targeting has yielded remarkable understanding of the role of several gene products in the OXPHOS system. We provide a "tool box" of mouse models with OXPHOS defects that could be used to answer diverse scientific questions.

Overexpressed mitochondrial leucyl-tRNA synthetase suppresses the A3243G mutation in the mitochondrial tRNA(Leu(UUR)) gene

The A3243G mutation in the human mitochondrial tRNA(Leu(UUR)) gene causes a number of human diseases. This mutation reduces the level and fraction of aminoacylated tRNA(Leu(UUR)) and eliminates nucleotide modification at the wobble position of the anticodon. These deficiencies are associated with mitochondrial translation defects that result in decreased levels of mitochondrial translation products and respiratory chain enzyme activities. We have suppressed the respiratory chain defects in A3243G mutant cells by overexpressing human mitochondrial leucyl-tRNA synthetase. The rates of oxygen consumption in suppressed cells were directly proportional to the levels of leucyl-tRNA synthetase. Fifteenfold higher levels of leucyl-tRNA synthetase resulted in wild-type respiratory chain function. The suppressed cells had increased steady-state levels of tRNA(Leu(UUR)) and up to threefold higher steady-state levels of mitochondrial translation products, but did not have rates of protein synthesis above those in parental mutant cells. These data suggest that suppression of the A3243G mutation occurred by increasing protein stability. This suppression of a tRNA gene mutation by increasing the steady-state levels of its cognate aminoacyl-tRNA synthetase is a model for potential therapies for human pathogenic tRNA mutations.

Twinkle mutations associated with autosomal dominant progressive external ophthalmoplegia lead to impaired helicase function and in vivo mtDNA replication stalling

Mutations in the mitochondrial helicase Twinkle underlie autosomal dominant progressive external ophthalmoplegia (PEO), as well as recessively inherited infantile-onset spinocerebellar ataxia and rare forms of mitochondrial DNA (mtDNA) depletion syndrome. Familial PEO is typically associated with the occurrence of multiple mtDNA deletions, but the mechanism by which Twinkle dysfunction induces deletion formation has been under debate. Here we looked at the effects of Twinkle adPEO mutations in human cell culture and studied the mtDNA replication in the Deletor mouse model, which expresses a dominant PEO mutation in Twinkle and accumulates multiple mtDNA deletions during life. We show that expression of dominant Twinkle mutations results in the accumulation of mtDNA replication intermediates in cell culture. This indicated severe replication pausing or stalling and caused mtDNA depletion. A strongly enhanced accumulation of replication intermediates was evident also in six-week-old Deletor mice compared with wild-type littermates, even though mtDNA deletions accumulate in a late-onset fashion in this model. In addition, our results in cell culture pointed to a problem of transcription that preceded the mtDNA depletion phenotype and might be of relevance in adPEO pathophysiology. Finally, in vitro assays showed functional defects in the various Twinkle mutants and broadly agreed with the cell culture phenotypes such as the level of mtDNA depletion and the level of accumulation of replication intermediates. On the basis of our results we suggest that mtDNA replication pausing or stalling is the common consequence of Twinkle PEO mutations that predisposes to multiple deletion formation.

Identification of novel mutations in five patients with mitochondrial encephalomyopathy

MELAS, MERRF, LHON and NARP, are well-established mitochondrial syndromes associated with specific point mutations of mitochondrial DNA (mtDNA). However, these recurrent mtDNA mutations account for only a minority of mitochondrial disease cases. To evaluate the impact of novel mtDNA mutations, we performed mtDNA sequence analysis in muscle and other tissues of 240 patients with different mitochondrial neuromuscular syndromes. We identified a total of 33 subjects with novel, private or uncommon mutations. Among these, five novel mutations were found in both paediatric and adult cases. We here report on the clinical description of these patients, as well as the biochemical and molecular genetic characterization of the corresponding mutations. Patients 1 and 2 showed changes in ND genes, patient 3 carried a heteroplasmic deletion in the COI gene, patients 4 and 5 carried heteroplasmic mutations in tRNA(Trp) and tRNA(Phe), respectively. Altogether, these data indicate that mtDNA analysis must become part of the routine screening for mitochondrial disorders.

How can we treat mitochondrial encephalomyopathies? Approaches to therapy

Mitochondrial disorders are a heterogeneous group of diseases affecting different organs (brain, muscle, liver, and heart), and the severity of the disease is highly variable. The chronicity and heterogeneity, both clinically and genetically, means that many patients require surveillance follow-up over their lifetime, often involving multiple disciplines. Although our understanding of the genetic defects and their pathological impact underlying mitochondrial diseases has increased over the past decade, this has not been paralleled with regards to treatment. Currently, no definitive pharmacological treatment exists for patients with mitochondrial dysfunction, except for patients with primary deficiency of coenzyme Q10. Pharmacological and nonpharmacological treatments increasingly being investigated include ketogenic diet, exercise, and gene therapy. Management is aimed primarily at minimizing disability, preventing complications, and providing prognostic information and genetic counseling based on current best practice. Here, we evaluate therapies used previously and review current and future treatment modalities for both adults and children with mitochondrial disease.

MitoP2: an integrative tool for the analysis of the mitochondrial proteome

Mitochondria are crucial for normal cell metabolism and maintenance. Mitochondrial dysfunction has been implicated in a spectrum of human diseases, ranging from rare monogenic to common multifactorial disorders. Important for the understanding of organelle function is the assignment of its constituents, and although over 1,500 proteins are predicted to be involved in mammalian mitochondrial function, so far only about 900 are assigned to mitochondria with reasonable certainty. Continuing efforts are being taken to obtain a complete inventory of the mitochondrial proteome by single protein studies and high-throughput approaches. To be of best value for the scientific community this data needs to be structured, explored, and customized. For this purpose, the MitoP2 database ( http://www.mitop2.de ) was established and is maintained in order to incorporate such data. The central database contains manually evaluated yeast, mouse, and human reference proteins, which show convincing evidence of a mitochondrial location. In addition, entries from genome-wide approaches that suggest protein localization are integrated and serve to compile a combined score for each candidate, which provides a best estimate of mitochondrial localization. Furthermore, it integrates information on the orthology between species, including Saccharomyces cerevisiae, mouse, human, Arabidopsis thaliana, and Neurospora crassa, thus mutually enhancing evidence across species. In contrast to other known databases, MitoP2 takes into account the reliability by which the protein is estimated as being mitochondrially located, as described herein. Multiple search functions, as well as information on disease causing genes and available mouse models, makes MitoP2 a valuable tool for the genetic investigation of human mitochondrial pathology.

Activation of the PPAR/PGC-1alpha pathway prevents a bioenergetic deficit and effectively improves a mitochondrial myopathy phenotype

Neuromuscular disorders with defects in the mitochondrial ATP-generating system affect a large number of children and adults worldwide, but remain without treatment. We used a mouse model of mitochondrial myopathy, caused by a cytochrome c oxidase deficiency, to evaluate the effect of induced mitochondrial biogenesis on the course of the disease. Mitochondrial biogenesis was induced either by transgenic expression of peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator alpha (PGC-1alpha) in skeletal muscle or by administration of bezafibrate, a PPAR panagonist. Both strategies successfully stimulated residual respiratory capacity in muscle tissue. Mitochondrial proliferation resulted in an enhanced OXPHOS capacity per muscle mass. As a consequence, ATP levels were conserved resulting in a delayed onset of the myopathy and a markedly prolonged life span. Thus, induction of mitochondrial biogenesis through pharmacological or metabolic modulation of the PPAR/PGC-1alpha pathway promises to be an effective therapeutic approach for mitochondrial disorders.

Optimized allotopic expression of the human mitochondrial ND4 prevents blindness in a rat model of mitochondrial dysfunction

Mitochondrial diseases due to mutations in mitochondrial DNA can no longer be ignored in most medical areas. With prevalence certainly higher than one in 6000, they probably represent the most common form of metabolic disorders. Despite progress in identification of their molecular mechanisms, little has been done with regard to therapy. We have recently optimized the allotopic expression for the mitochondrial genes ATP6, ND1, and ND4 and obtained a complete and long-lasting rescue of mitochondrial dysfunction in the human fibroblasts in which these genes were mutated. However, biosafety and benefit to mitochondrial function must be validated in animal models prior to clinical applications. To create an animal model of Leber Hereditary Optic Neuropathy (LHON), we introduced the human ND4 gene harboring the G11778A mutation, responsible of 60% of LHON cases, to rat eyes by in vivo electroporation. The treatment induced the degeneration of retinal ganglion cells (RGCs), which were 40% less abundant in treated eyes than in control eyes. This deleterious effect was also confirmed in primary cell culture, in which both RGC survival and neurite outgrowth were compromised. Importantly, RGC loss was clearly associated with a decline in visual performance. A subsequent electroporation with wild-type ND4 prevented both RGC loss and the impairment of visual function. Hence, these data provide the proof-of-principle that optimized allotopic expression can be an effective treatment for LHON, and they open the way to clinical studies on other devastating mitochondrial disorders.

Maternally inherited diabetes mellitus, deafness, chronic progressive external ophthalmoplegia and myopathy as the result of A3243G mutation of mtDNA

A mitokondriális encephalomyopathiát, laktátacidózist, stroke-szerű epizódokat (MELAS-szindrómát) a leggyakrabban a mitokondriális genom A3243G-mutációja okozza. A mitokondriális DNS (mtDNS) A3243G-szubsztitúciója számos egyéb klinikai tünet, tünetcsoport hátterében is előfordulhat. Jelen közleményünkben egy 33 éves nő esetét ismertetjük, akinél a serdülőkori ophthalmoplegia externa, 19 éves korban, szülést követően jelentkező generalizált izomgyengeség, terhelési intolerancia, progresszív hypacusis és diabetes mellitus hátterében a szövettani vizsgálat mitokondriális betegséget igazolt. A genetikai analízis az mtDNS tRNS Leu(UUR) génjében heteroplazmikus formában A3243G-cserét talált. Esetünkben a klasszikus MELAS-fenotípus nem jelentkezett, az irodalmi adatok alapján azonban az A3243G-mutációhoz izoláltan társuló maternális öröklődésű diabetes mellitus, progresszív nagyothallás, krónikus progresszív ophthalmoplegia externa (CPEO), terhelési intolerancia és myopathia együttesen fordult elő. A családon belüli fenotípus egységes, a proband szintén A3243G-mutációt hordozó leányának is ptosisa, terhelési intoleranciája és myopathiája van. Az esetleírás kapcsán rövid áttekintést nyújtunk az A3243G-mutációkhoz társuló változatos klinikai fenotípusokról, valamint a CPEO-szindróma hátterében álló mtDNS-alterációkról.

Pathogenic mitochondrial DNA mutations are common in the general population

Mitochondrial DNA (mtDNA) mutations are a major cause of genetic disease, but their prevalence in the general population is not known. We determined the frequency of ten mitochondrial point mutations in 3168 neonatal-cord-blood samples from sequential live births, analyzing matched maternal-blood samples to estimate the de novo mutation rate. mtDNA mutations were detected in 15 offspring (0.54%, 95% CI = 0.30–0.89%). Of these live births, 0.00107% (95% CI = 0.00087–0.0127) harbored a mutation not detected in the mother's blood, providing an estimate of the de novo mutation rate. The most common mutation was m.3243A→G. m.14484T→C was only found on sub-branches of mtDNA haplogroup J. In conclusion, at least one in 200 healthy humans harbors a pathogenic mtDNA mutation that potentially causes disease in the offspring of female carriers. The exclusive detection of m.14484T→C on haplogroup J implicates the background mtDNA haplotype in mutagenesis. These findings emphasize the importance of developing new approaches to prevent transmission.

Mitochondrial ATP synthase disorders: molecular mechanisms and the quest for curative therapeutic approaches

In mammals, the majority of cellular ATP is produced by the mitochondrial F1F(O)-ATP synthase through an elaborate catalytic mechanism. While most subunits of this enzymatic complex are encoded by the nuclear genome, a few essential components are encoded in the mitochondrial genome. The biogenesis of this multi-subunit enzyme is a sophisticated multi-step process that is regulated on levels of transcription, translation and assembly. Defects that result in diminished abundance or functional impairment of the F1F(O)-ATP synthase can cause a variety of severe neuromuscular disorders. Underlying mutations have been identified in both the nuclear and the mitochondrial DNA. The pathogenic mechanisms are only partially understood. Currently, the therapeutic options are extremely limited. Alternative methods of treatment have however been proposed, but still encounter several technical difficulties. The application of novel scientific approaches promises to deepen our understanding of the molecular mechanisms of the ATP synthase, unravel novel therapeutic pathways and improve the unfortunate situation of the patients suffering from such diseases.

The optimized allotopic expression of ND1 or ND4 genes restores respiratory chain complex I activity in fibroblasts harboring mutations in these genes

Leber's Hereditary Optic Neuropathy (LHON) was the first maternally inherited mitochondrial disease identified and is now considered the most prevalent mitochondrial disorder. LHON patients harbor mutations in mitochondrial DNA (mtDNA). In about 90% of cases, the genes involved encode proteins of the respiratory chain complex I. Even though the molecular bases are known since 20 years almost all remains to be done regarding physiopathology and therapy. In this study, we report a severe decrease of complex I activity in cultured skin fibroblasts isolated from two LHON patients harboring mutations in ND4 or ND1 genes. Most importantly, we were able to restore sustainably (a) the ability to grow on galactose, (b) the ATP synthesis rate and (c) the complex I activity, initially impaired in these cells. Our strategy consisted of forcing mRNAs from nuclearly-encoded ND1 and ND4 genes to localize to the mitochondrial surface. The rescue of the respiratory chain defect observed was possible by discreet amounts of hybrid mRNAs and fusion proteins demonstrating the efficiency of their mitochondrial import. Hence, we confirmed here for two mitochondrial genes located in the organelle that the optimized allotopic expression approach represents a powerful tool that could ultimately be applied in human therapy for LHON.

What causes mitochondrial DNA deletions in human cells?

Mitochondrial DNA (mtDNA) deletions are a primary cause of mitochondrial disease and are likely to have a central role in the aging of postmitotic tissues. Understanding the mechanism of the formation and subsequent clonal expansion of these mtDNA deletions is an essential first step in trying to prevent their occurrence. We review the previous literature and recent results from our own laboratories, and conclude that mtDNA deletions are most likely to occur during repair of damaged mtDNA rather than during replication. This conclusion has important implications for prevention of mtDNA disease and, potentially, for our understanding of the aging process.

A mouse model of mitochondrial disease reveals germline selection against severe mtDNA mutations

The majority of mitochondrial DNA (mtDNA) mutations that cause human disease are mild to moderately deleterious, yet many random mtDNA mutations would be expected to be severe. To determine the fate of the more severe mtDNA mutations, we introduced mtDNAs containing two mutations that affect oxidative phosphorylation into the female mouse germ line. The severe ND6 mutation was selectively eliminated during oogenesis within four generations, whereas the milder COI mutation was retained throughout multiple generations even though the offspring consistently developed mitochondrial myopathy and cardiomyopathy. Thus, severe mtDNA mutations appear to be selectively eliminated from the female germ line, thereby minimizing their impact on population fitness.

Age-associated mosaic respiratory chain deficiency causes trans-neuronal degeneration

Heteroplasmic mitochondrial DNA (mtDNA) mutations (mutations present only in a subset of cellular mtDNA copies) arise de novo during the normal ageing process or may be maternally inherited in pedigrees with mitochondrial disease syndromes. A pathogenic mtDNA mutation causes respiratory chain deficiency only if the fraction of mutated mtDNA exceeds a certain threshold level. These mutations often undergo apparently random mitotic segregation and the levels of normal and mutated mtDNA can vary considerably between cells of the same tissue. In human ageing, segregation of somatic mtDNA mutations leads to mosaic respiratory chain deficiency in a variety of tissues, such as brain, heart and skeletal muscle. A similar pattern of mutation segregation with mosaic respiratory chain deficiency is seen in patients with mitochondrial disease syndromes caused by inherited pathogenic mtDNA mutations. We have experimentally addressed the role of mosaic respiratory chain deficiency in ageing and mitochondrial disease by creating mouse chimeras with a mixture of normal and respiratory chain-deficient neurons in cerebral cortex. We report here that a low proportion (>20%) of respiratory chain-deficient neurons in the forebrain are sufficient to cause symptoms, whereas premature death of the animal occurs only if the proportion is high (>60–80%). The presence of neurons with normal respiratory chain function does not only prevent mortality but also delays the age at which onset of disease symptoms occur. Unexpectedly, respiratory chain-deficient neurons have adverse effect on normal adjacent neurons and induce trans-neuronal degeneration. In summary, our study defines the minimal threshold level of respiratory chain-deficient neurons needed to cause symptoms and also demonstrate that neurons with normal respiratory chain function ameliorate disease progression. Finally, we show that respiratory chain-deficient neurons induce death of normal neurons by a trans-neuronal degeneration mechanism. These findings provide novel insights into the pathogenesis of mosaic respiratory chain deficiency in ageing and mitochondrial disease.